The present invention relates to a storage battery cell based on the principle of the oxygen cycle.
German Patent Specification 27 42 869 discloses a square nickel/cadmium cell in button form,
having a block-shaped, sealed gastight polymeric cell housing; PA0 having a few essentially rectangular electrode plates lined up to form an electrode stack and having different polarity, with separators inserted in between in each case; PA0 furthermore having positive and negative terminal leadthroughs which are passed through the wall of the same side of the housing and formed as contact discs integrated in the wall and which are in each case connected in an electrically conducting manner to current collector lugs of appropriate polarity; PA0 the electrode plates being arranged parallel to the largest housing wall of the terminal leadthroughs and all the electrode plates having in each case a central hole, formed and arranged coincidentally, for the passage of a stud which joins the two oppositely situated, largest housing walls together in a tension-proof manner; PA0 the positive electrode plates furthermore being provided in each case with a current collector lug mounted at the edge and the negative electrode plates being provided in each case with a current collector lug which is likewise mounted at the edge, but is arranged in an offset manner with respect to the positive current collector lugs and electrically insulated from them.
Apart from its low storage capacity due to the specific button design cited here, a further disadvantage (even in the case of larger, higher-capacity designs for this type storage battery) of the conventional nickel/ cadmium cell design (which can be recognized, for example, in German Patent Specification 40 41 123), is that the service life of the cells is not optimum. The cause of the less than adequate service life is the sensitive electrolyte balance of the low-electrolyte system: in order to be able to exhaust the capacity of the electrodes, on the one hand more than 90% of the pore volume of the electrodes must be filled with electrolyte, and on the other hand, for good charge ability, sufficiently large, electrolyte-wetted, accessible recombination areas must be available for oxygen reduction. The electrolyte can therefore only partly fill the free pore volume of the electrode stack components. The amount of electrolyte in the cell must be determined precisely and maintained, even taking into account changes in the porosity of the electrodes during the service life of the cell. Unavoidable thickness tolerances in the electrode stack components result in differing degrees of compression during their installation in the housing (which of course has predetermined dimensions), and consequently also in different electrolyte absorption in the electrode stack. There is thus a danger that the separators, as the most pliable component in the stack, become compressed and lose electrolyte, so that the resistance increases. A loose packing of the electrode stack, on the other hand, results in an imperfect contact between separator and electrodes, and a high resistance and unequal charge distribution in the electrodes, even in the case of microporous capillary-active separators. Finally, a desired homogeneous electrolyte distribution in large-area separators which are made of polymeric nonwoven fabric and whose capillary activity is usually achieved by wetting agents which are not completely resistant to aging, cannot be reliably achieved in cells of conventional construction. Pressure differences between the internal cell pressure and atmospheric pressure during cycling are transmitted via the cell housing walls to the electrode stack and also result in an undesirable alteration in the stack compression pressure.
Further disadvantages of conventionally constructed nickel/cadmium cells reside in the poorly defined compression state of the electrode stack components, in particular of the separators, which can be most easily compressed, and consequently in their degree of electrolyte filling, and, particularly in the case of large cells, in the imperfect electrolyte distribution in the electrode stack components, specifically, again, in the separators. Although gas diffusion structures offer spare volume for the electrolyte, the rate of oxygen reduction is strongly dependent on their degree of electrolyte filling; the electrolyte absorption capacity of the diffusion structures can therefore only be utilized to a limited extent. Pressure differences between the internal cell pressure and atmospheric-pressure during cycling are transmitted via the cell housing wall to the electrode stack and result in undesirable alterations in the stack compression pressure. In cells of conventional construction, it is precisely the largest face of the housing which presses perpendicularly on the electrode assembly. The bowing of this face of the housing is therefore also the greatest and, consequently, an equal distribution of the compression pressure is most difficult to achieve. The reproducibility of the cells suffers due to the unavoidable thickness tolerances of the stack components, which, as a stack assembly, are pushed into a housing with substantially identical dimensions. The resulting different compression of the stack components, in particular of the separators, requires either an individual adjustment of the quantity of electrolyte (which is in any case difficult to determine), or an acceptance of greater tolerances in the permissible charging current.
Finally, thickness and pore volume of the components alter with service life; in particular, the positive electrodes tend to swell. The separators may therefore be considerably crushed, at least in the vicinity of the housings, which are rigid at the corners, as a result of which electrolyte is pressed out and, in extreme cases, short circuits are produced.
The object of the present invention therefore is to improve the storage battery cells of this basic type so as to provide a longer service life and a better electrolyte distribution.
Proceeding from the acknowledged prior art, this object is achieved by the novel cell structure according to the invention. This structure avoids all the disadvantages of the prior art by utilizing square electrode stack elements having a central hole, which are stacked along an imaginary axis through the central hole, to form a prismatic stack which is higher than the side length of the components. A spring element (for example a slotted spring leaf, an elastomeric cushion or a helical spring assembly), arranged between the cell housing base and the stack essentially fills the cell cross section, and exerts a defined force on the electrode stack. The change in stack height is thus limited to not more than 5% of the stack height. The positive and negative current collector lugs are insulated from one another in the central hole by a separating element, and are routed parallel to the terminals in the housing lid. Overlapping separator edges between stack and housing act as a wall wick and support the housing walls against the atmospheric pressure.
Due to defined axial compression of the electrode stack by means of a face-covering arrangement of the spring elements (which have a moderate pressure that changes only relatively little within the utilizable spring travel), the sensitive electrolyte balance of the cell is affected to no more than a readily tolerable extent, even after a prolonged period of use with gradually swelling electrode plates or during charging and discharging of the cell. A supply of electrolyte which is approximately equal at all the required points can be ensured at all times due to an exchange of electrolyte which is accelerated at all four outer peripheral sides and in the vicinity of the central duct formed by the central holes.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.